Total air/gas premix burners are currently known to be widely employed to produce thermal energy in gas boilers.
The use of these burners is becoming rapidly more common, replacing the traditional atmospheric burners because with respect to the latter they allow to:
[A] have lower polluting substance emissions (nitrogen and carbon oxides);
[B] have high thermal exchange efficiencies at all thermal power rates, and in particular at minimum thermal power; and
[C] have high modulation ranges between maximum and minimum thermal power of the burner.
At present premix air/gas burners are mainly made using the following essential components:
a fan for supplying the air/gas mixture to a combustion head;
a “pneumatically” actuated gas valve provided with a flow regulator;
an air/gas mixing system consisting of a venturi channel or diaphragm having a similar function (see below); and
a combustion head provided with a device for igniting the air/gas mixture combustion.
In these systems, the “active device” (also called “driver”) is represented by the fan which, electrically fed in an appropriate manner, provides comburent air to the burner in amount directly proportional to the thermal power that is intended to be provided to the burner, and thus to the thermal power of the burner head.
The passive device (also called “follower”) is represented by the gas valve, which is capable of providing gas in amount directly proportional to the amount of air blown into the system by virtue of the regulation system illustrated thereinafter.
Gas valves are normally characterized in that, independently from the inlet gas pressure (obviously within the working limits allowed by the valve itself and corresponding to the network gas distribution pressures), they provide output gas at a pressure equal to the pressure exerted on their “regulator” except for a difference called “offset” value, adjustable by acting on the valve. In order to expand the modulation range of premix burners of traditional type, the Applicant designed a premix burner of new concept, which was object of international application WO2009/0133451 in the Applicant's name.
Although the results obtained by the premix burner object of international application WO2009/0133451 were overall satisfactory, the reduction of deleterious effects consequent to offset variations which may occur in the gas valve during its long working time was not found optimal.
The present premix burner was designed to solve these drawbacks and must be considered as a further evolution of the premix burner described and claimed in aforesaid international application WO2009/0133451.
The minimum thermal flow rate, i.e. the flow rate in which offset variations of the gas valve correspond to greater air/gas ratio variations, will be taken as reference in order to explain the behavior of the system described in international application WO2009/0133451.
If offset is negative, the gas pressure at the end of the gas feeding pipe is lower than the air pressure at the venturi channel inlet.
Therefore, due to a given air overpressure, there is a passage of air through the nozzle of the venturi channel intersected by the plug, air which enters into the common segment of the gas circuit and dilutes the gas which is entering through the nozzle of the venturi channel free from plug.
Conversely, if offset is positive, the pressure in the common segment of the gas circuit is higher than the air pressure at venturi channel inlet.
Therefore, there is a passage of gas through the venturi channel nozzle intercepted by the plug, gas which enters into the air inlet segment in common to both venturi channels increasing the amount of gas which enters into the venturi channel free from plug.
Finally, if the system works in ideal reference condition, with offset=0 Pa, the air pressures at venturi channel inlet and in the common gas circuit are equal.
Therefore, there is no passage of neither air nor gas through the nozzle of the venturi channel intercepted by the plug and the air/gas ratio will be maintained constant at reference value.
Independently from the offset value set in the gas valve, as the air/gas mixture flow rate aspirated by the fan increases the plug starts opening allowing also its venturi channel to generate vacuum, gradually attenuating the phenomena illustrated above to cancel them out completely and to provide its contribution of the air/gas mixture flow rate with respective ratio values always closer to the reference value which is found at maximum thermal flow rate.